Electrostatographic printing operates according to the principles and embodiments of non-impact printing as described, e.g., in "Principles of Non-Impact Printing" by Jerome L Johnson (1986) - Palatino Press - Irvine Calif., 92715 USA).
Electrostatographic printing includes electrographic printing in which an electrostatic charge is deposited image-wise, e.g. by ionography, on a dielectric recording member as well as electrophotographic printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on the recording member. "Direct" development is a positive-positive development, and is particularly useful for reproducing pictures and text. "Reversal" development is of interest in or when from a negative original a positive reproduction has to be made or vice-versa, or when the exposure derives from an image in digital electrical signal form, wherein the electrical signals modulate a laser beam or the light output of light-emitting diodes (LEDs). It is advantageous with respect to a reduced load of the electric signal modulated light source (laser or LEDs) to record graphic information (e.g. printed text) in such a way that the light information corresponds with the graphic characters so that by "reversal" development in the exposed area of a photoconductive recording layer, toner can be deposited to produce a positive reproduction of the electronically stored original. In high speed electrostatographic printing the exposure derives practically always from electronically stored, i.e. computer stored information.
In the electrophotographic art, an electrostatographic single-pass multiple station multi-color printer is known, in which an image is formed on a photoconductive belt and is then transferred to a paper receiving sheet or web whereon the toner image is fixed, whereupon the web is usually cut into sheets containing the desired print frame.
In an alternative printer, toner images are transferred to an insulating belt from distinct image forming stations and are then transferred to the receiving sheet or web and fixed thereon.
In U.S. Pat. No. 3,694,073 (Bhagat/Xerox Corporation), there is described an electrostatographic printer for forming an image onto a web. The printer comprises a plurality of toner image-producing stations each comprising a photoconductive drum as an electrostatic image element, onto which a toner image can be formed, means for forming an electrostatic latent image on each drum and a developing unit for depositing toner onto the electrostatic latent image to render the image visible and transferable. The printer further includes means for conveying a web past the image-producing stations and transfer means for transferring the toner image on the drum onto the web.
In a printer in which the developing unit brings toner particles into contact with the electrostatic imaging element, for example in the form of a drum or an endless belt, there will still be contact with the toner in the non-printing stage and toner adhering to the imaging element web becomes transferred to the receptor element as the printing process is started, resulting in smudging on the first few prints. This problem occurs for example when the printer is not printing for a certain period of time.
We have discovered that a solution to this problem is provided if the developing unit is selectively moveable between an operative position and a non-operative position while the drum rotates, thereby enabling the developing unit to be located in a position out of contact with the drum when the printer is not in use.